Holding furnaces



Filed Aug. 16, 1962 3 Sheets-Sheet 1 Fig.1

INVENTOR ER\K ALLAN OLSS ON ATTORNEYS March 23, 1965 E. A. OLSSON 3,174,737

HOLDING FURNACES Filed Aug. 16, 1962 3 Sheets-Sheet 2 iNVENTOR EPAK ALLAN OLSSON BY 011d ATTORNEY$ March 23, 1965 E. A. OLSSON 3,174,737

HOLDING FURNACES Filed Aug. 16, 1962 3 Sheets-Sheet 3 Fig.

INVENTOR EPJK ALLAN OLSSON BY WWW ATTORNEYS United States Patent 3,174,737 HOLDING FURNAQES Erik Allan @lsson, Zurichstrasse 66, Kusnacht, Zurich, Switzerland Filed Aug. 16, 1962, Ser. No. 217,328 4 Claims. (Cl. 266-33) This invention relates to an improvement in holding furnaces which are designed as bricked enclosures on the side of which is at least one heating unit to transfer heat to the melt in the furnace. The invention especially comprises a device for use with holding furnaces on the side of which is at least one induction heating unit, that is to say, a melting trough heated by induction.

The purpose of the invention is to make it possible to use holding furnaces of the type described above in connection with fully continuous casting of iron and steel, without risk of breakout of the melt through the induction heating units and without the other difliculties which have previously been considered unavoidable under such conditions, as will be discussed subsequently at greater length. Moreover the invention is intended to make possible the attainment of several advantages as regards simple installation and simple maintenance, even during operation, operational economy and homogeneous analy sis of the melts.

To accomplish these purposes this invention is particularly characterized in that the heat generating unit can be rotated above the surface of the melt while pouring from the furnace proceeds.

The following, which should be read with reference to the appended drawings, will further clarify the invention.

FIG. 1 is a side View of a furnace designed in accordance with this invention, in which the pear-shaped furnace enclosure is shown in longitudinal section and the induction heating unit is shown rotated down beneath the surface of the melt.

FIG. 2 shows a cross section through the furnace at IIII in FIG. 1, with the induction heating unit also rotated below the surface of the melt.

FIG. 3 shows a cross section through the furnace at II-II in FIG. 1 in which the induction heating unit is shown rotated above the surface of the melt and is thus disconnected: during transportation the furnace enclosure is in this position.

FIG. 4 is a diagram showing various temperature curves.

Before presenting a detailed description of the invention it will be useful to discuss briefly the previous state of the art, making references to FIG. 4. In this diagram the temperature of the melt is plotted as the ordinate while the casting period is plotted as abscissa. The two horizontal broken lines indicate the maximum practical pouring and the minimum permissible pouring temperature.

Most presently known continuous casting processes for iron and steel make use of a suitable method of pouring from a ladle with bottom-pour hole and stopper, or lippouring from a ladle (ordinarily provided with a siphon) and usually held at high temperature with an oil burner.

The choice of method for transferring the melt to the actual casting equipment is determined primarily by the length of period required for pouring and the pouring rate. The softening of the stopper rod that occurs as time elapses limits the use of ladles with stoppers to fairly short casting periods and the excessive risks for both clogging of the bottom-pour hole (freezing) and the break-down of stopper and stopper rod as a result of too frequent open ing and closing of the hole. Lip pouring from tilting ladles is most often used when pouring with ladle and stopper is considered less suitable. Since better conditions for effective heating exist, longer casting periods 3,174,737 Patented Mar. 23, 1965 "ice can be achieved. However, an oil burner provided with neither pre-heated nor oxygen-enriched air provides too little additional heat to compensate entirely for heat losses in the ladle, since the temperature difference between the flame and metal bath is fairly small and as a result the amount of heat transferred is low. Thus, by means of this process, the casting period has been prolonged only moderately as compared with the casting periods achieved with bottom-pour ladies equipped with stopper rods. If the air is pre-heated and/or enriched with oxygen, thus increasing the temperature of the oil flame, it will reduce prohibitively the service life of the ladle lining. Since casting periods are thus limited for the above reasons, it is necessary to choose a sufficiently large number of castings or casting units that the ladles can be emptied before the temperature of the melt has time to drop below certain critical values for each grade of steel, in spite of the fact that a longer casting period and fewer ladles would be otherwise preferable.

For casting metals with lower melting points than iron and steel special holding furnaces have already found practical application, which has made possible the introduction of a completely continuous casting process, which contrasts with the previously used batchwise casting methods for iron and steel. Such holding furnaces, often designed as bricked enclosures on the sides of which are mounted so-called induction heating units, i.e. a melting trough or launder heated by induction, have not been considered usable for iron and steel as a result of the relatively high temperature, which causes great danger of chemical attack on the brickwork and the accompanying risk for breakout of the melt through the induction heating units.

Another disadvantage put forth for furnaces of this type is that the induction heating units cannot be emptied of their contents between pouring. The molten metal left behind is thus considered able to affect the analysis of the next charge.

There is also a prevailing opinion that casting periods longer than 1 or 1.5 hours should not be attempted when casting iron and steel since the charge analysis would change as a result of reactions with the brick-work. It has, however, been shown that the analysis does not change significantly if the melt and the slag are not exposed to the oxidizing effects of the atmosphere.

Furthermore induction furnaces of the crucible type must be filled to a minimum level before suflicient heat is generated in the melt to compensate for heat losses, so that the temperature of the melt is rapidly lowered when the furnace is tilted beyond a certain angle, i.e. when the furnace contents are emptied beyond a certain limit.

It should be further mentioned that the installation costs for electrically heated holding furnaces have heretofore been discouragingly high. Power requirement calculations are based on the assumption that the entire contents of the furnace shall be held at constant temperature and also that a fast temperature correction is desirable for the entire charge, especially with regard to the fairly high temperature drops which occur when the melt is tapped from the melting furnace to the holding furnace via a transfer ladle. As an example of the power required for an installation calculated on this basis it can be stated that a 30-ton charge can lose from 25-30 C. when tapped into a ladle which was pro-heated to 800 C. and

-' that approximately the same temperature drop occurs when the holding furnace is filled. However, losses in the ladle or holding furnace can be calculated to be the equivalent to a power loss of less than 6 kw. per ton of melt, after thermal equilibrium is achieved between the melt and the brickwork. Of course the heat transferred to the brickwork is highly dependent upon its initial temperature.

aways? To compensate for heat losses when pouring from the transfer ladle to the holding furnace, at temperature loss calculated to be 25 C, the power installation must deliver at least 600 kw. if the melt is to be restored in 15 minutes to the original temperature it had in the ladle, which in this example is assumed to be the desired pouring temperature. The avoidance of one of these pourings, i.e. elimination of the transfer ladle by replacing it with a holding furnace which is so designed that the melt can be tapped into it directly from the melting furnace would reduce substantially the power required. Furthermore it is quite evident that the power required drops as furnace contents are reduced, i.e., as time elapses after pouring has commenced. While the temperature fall of the melt during this cycle would approximate curve A, FIG. 4, if no heat was, added, 240 kw. of added power would result in a successively rising temperature in the melt which would approximate curve B, FIG. 4. A substantially lower power installation, for example, roughly equivalent to the power needed to cover the total heat losses just before curve A in FIG. 4 crosses the line representing the lowest permissible pouring temperature, would be sufficient to prevent the interruption of pouring due to an excessively cool melt. The temperature cycle would then, in principle, approximate curve C in FIG. 4. Such a temperature cycle is possible if a holding furnace designed in accordance with this invention is used for continuous casting of iron and steel.

In the version of this invention chosen for descriptive purposes it comprises a holding furnace in which the furnace enclosure is mounted in bear'mgs so that it can be rotated by frame 12, which can be tilted by means of a piston and cylinder arragnement ll, thus making it possible to pour the melt through tap hole 13. The furnace enclosure 10 is rotated by motor 14 via worm gear 15 which is self-braking and can thus without any special built-in brake permitany desirable angle of rotation for furnace enclosure 10, regardless of the angle of tilt of frame 12. Bearings 16 and 17, which make it possible for the furnace enclosure 1% to rotate, can be opened to enable the enclosure 10 to be lifted free for transportation and to be filled directly from the melting furnace.

The illustrated version of the holding furnace is provided with an induction heating unit 18, which is arranged so as to heat, by induction heating, that portion of the melt which flows down into melting trough 19, designed as a projection on the wall of the furnace enclosure 19. Furnace enclosure 16 is provided with a charging hole 24), FIGS. 2 and 3. The induction heating unit 18 and the charging hole 29 are positioned peripherally on the furnace enclosure 10, FTGS. 2 and 3, so that the induction heating unit 18 can be rotated above the surface of the melt 21, FIG. 3, without the melt running out through the charging hole 29. Naturally several induction heating units could be positioned adjacent to each other, for example in such a manner that one induction heating unit would be located immediately adjacent to tap hole 13, so that the melted metal heated in this induction heating units melting loop would be poured through tap hole 13 when the furnace was tilted while the other induction heating units would be located obliquely behind the first and staggered in relation to each other so that one unit after the other would rise successively above the surface of the melt 21, as determined by the amount poured from the furnace enclosure 10 and the angle of rotation of the furnace enclosure 10.

If when using several induction heating units, a partition provided with a hole is built in the furnace enclosure between the induction heating unit lying nearest the tap hole and the other induction heating units the temperature of the molten metal being poured can be quickly regulated, while the remaining contents of the furnace enclosure 10 require only as much power as needed to keep the melt in the liquid state. Thus thepower needed by the installation can be kept fairly low, since casting can commence immediately, even if the temperature of the melt is too low when the furnace is charged, since the temperature of entire contents of the furnace need not be raised in a short interval, but only that portion of the melt which continuously flows past the induction heating unit located immediately adjacent to tap hole 13.

It is, of course, conceivable that heat generating units other than the induction heating units described above could be used; for example, types based on resistance heating of the melt 21. Similarly, furnace enclosure 10 can have another shape and the heat generating unit can be located differently. Thus, in cases where the slag at the surface of the melt 21 does not need to be prevented from following the melt during pouring by means of a slag-retaining bridge or siphon, a single hole would serve as both filling hole and tap hole.

As an example of the advantages of the described holding furnace can be mentioned its use in continuous casting, as illustrated by the schematically shown continuous casting equipment 22, FIG. 1, particularly when charges are large and the cast billet is small, i.e. requires a relatively small quantity of melted metal per casting and unit of time. Assuming that steel from a 60-ton open-hearth furnace which is tapped every 7 or 8 hours is to be poured to form a square x 70 mm. ingot, the equivalent of approximately 200 kg. per min., i.e. at least 6 parallel continuous castings would be required if an ordinary bottom-pour ladle was used, with a maximum casting period of 50 minutes. If the casting machine could pour steel from only a single furnace the utilization of capacity would be extremely low. With the introduction of an oil-fired holding ladle it would be, at best, possible to reduce the number of castings to 4, thus increasing the casting period to approx. minutes. The ideal solution in this case would be to be able to pour to a single casting, i.e. continuously for approx. 5 hours, which would require an effective holding furnace. The use of electric arc heating appears unadvisable due to the risk of carburizing; as a result inductive heating is desirable. A conventional crucible-type furnace of said size has not as yet been built and would, at a certain angle of tilt (estimated as an angle which would leave 15 tons of melt remaining in the crucible), fail to provide sufiicient additional heat. Transferring steel from the melting furnace to the holding furnace by means of a transfer ladle would in most cases be necessary, not least because of the fact that cooling water must be supplied to the induction as soon as the crucible contains melted metal. The holding furnace comprising this invention on the other hand can be designed, as mentioned above, as a movable furnace, thus halving pouring losses. When moved, the induction heating unit 18, 19 is rotated up over the melt surface and cooling water therefore need not be connected up before the furnace is replaced in its tilting frame 12 at the casting machine. If a check of the melt temperature reveals that heat must be added, the induction heating unit 18, 19 or the induction heating units can be rotated beneath the surface of the melt and connected up, as a rule, only after cooling water and electric power lines (not shown) are connected to the furnace, which can be accomplished by quick couplings (electric power can be connected up via knife switches).

In summary it can be said that the following special advantages are achieved by this invention:

(a) Breakout of the melt through the inductance heating units is avoided, since by means of special indicating devices (cooling water thermometers) the brickwork has been proven to have a sufficient minimum thickness;

(b) The holding furnace can be made movable due to the fact that cooling water otherwise needed for the heat generating unit can be shut off and disconnected;

(c) It is possible to replace a worn induction heating unit even while the furnace contains molten metal;

(d) It is possible to empty the induction heating units between pourings:

III

(2) It is possible to shut off one or more of the incluction heating units as the furnace is emptied.

I claim:

1. A holding furnace for casting molten metals, comprising a metal enclosure lined with refractory material, a pair of oppositely disposed trunnion-like projections defining an axis of rotation for said enclosure, a pair of bearings journaling said projections for rotatably mounting the enclosure, at least one heating unit secured to the underside of the enclosure to transfer heat thereto, a melt discharge opening passing through one of said projections, means for rotating said enclosure to elevate the heating unit above said axis and means for tilting the enclosure about a point lying in said axis so as to elevate the surface level of a melt above said discharge opening.

2. A holding furnace according to claim 1 wherein each of said bearings is provided With a removable cap portion, whereby the enclosure may be lifted from the bearings and transported to another location for charging the furnace.

3. A holding furnace according to claim 1 wherein said means for tilting the enclosure comprises a frame mounting the enclosure and said bearings, a pivot at one end of the frame pivotally supporting one of said bearings, and fluid pressure means for elevating the other end of the frame.

4. A holding furnace according to claim 1 wherein is provided a plurality of heating units one being immediately adjacent said discharge opening, and a partition in said enclosure separating the heating unit adjacent the discharge opening from the others, said partition having at least one opening therethrough.

References Cited by the Examiner UNITED STATES PATENTS 2,474,443 6/49 Tame. et a1. 2,494,501 1/50 Bahney et al. 2,707,720 5/55 Tama. 1333 MORRIS O. WOLK, Primary Examiner.

JAMES H. TAYMAN, JR., Examiner. 

1. A HOLDING FURNANCE FOR CASTING MOLTEN METALS, COMPRISING A METAL ENCLOSURE LINED WITH REFRACTORY MATERIAL, A PAIR OF OPPOSITELY DISPOSED TRUNNION-LIKE PROJECTIONS DEFINING AN AXIS OF ROTATION FOR SAID ENCLOSURE, A PAIR OF BEARINGS JOURNALING SAID PROJECTIONS FOR ROTATABLY MOUNTING THE ENCLOSURE, AT LEAST ONE HEATING UNITS SECURED TO THE UNDERSIDE OF THE ENCLOSURE TO TRANSFER HEAT THERETO, A MELT DISCHARGE OPENING PASSING THROUGH ONE OF SAID PROJECTIONS, MEANS FOR ROTATING SAID ENCLOSURE TO ELEVATE THE . HEATING UNIT ABOVE SAID AXIS AND MEANS FOR TILTING THE ENCLOSURE ABOUT A POINT LYING IN SAID AXIS SO AS TO ELEVATE THE SURFACE LEVER OF A MELT ABOVE SAID DISCHARGE OPENING. 